Optical imaging of deep tissue is used to probe structures within biological specimens for laboratory research and biomedical purposes. This includes the imaging of internal organs and subdermal tissue in animals such as mice, zebrafish, or humans, and one of the goals is to learn about internal structures without surgery or other intrusive measures.
In one technique of deep tissue imaging, fluorescent agents which are associated with a specific target in the specimen are imaged by exciting them with illumination light, causing them to fluoresce; the fluorescent emission is separated from the illumination light, which has a different wavelength, by barrier filters and then is detected using a very sensitive camera such as a cooled CCD detector. In other techniques, the specimen is modified using agents that cause it to produce material that is inherently fluorescent, with the most common example being green fluorescent protein (GFP). Further techniques involve use of quantum dots as luminous probes.
As used herein, compounds such as fluorescent dyes, fluorescent proteins such as GFP, quantum dots, surface-enhanced Raman reagents, as well as related compounds or others used for similar purposes, are all examples of a “target compound” of a measurement.
The signals produced in such experiments are typically weak. In general, robust detection of the weak levels of light emitted from the deep structures is beneficial because it provides earlier, or more reliable, detection of the structures being studied. Also, it may enable detection of lower levels of the target compound. Accordingly, techniques or apparatus used for deep tissue imaging are valued if they offer a low detection threshold.